1. Field of the Invention
The present invention relates to an optical apparatus such as a digital still camera, a video camera, and an interchangeable lens, and more particularly, to a structure for preventing a cam engaging portion for driving a lens barrel of the optical apparatus from being disengaged when an external force is applied to the lens barrel.
2. Related Background Art
In one basic construction of a lens barrel that expands and contracts in an optical axis direction, a cam follower portion provided on a cam barrel for cam-driving a lens holding barrel for holding a lens is engaged with a cam groove portion provided in a fixed barrel, and the cam barrel rotates around the optical axis and is thereby driven in the optical axis direction, so the lens holding barrel is also driven in the optical axis direction.
In the lens barrel described above, when an external force is applied to a front end of the lens barrel, for example, when the optical apparatus is dropped, the cam follower portion may be disengaged from the cam groove portion to make it impossible to drive the lens barrel.
In view of the foregoing circumstances, an optical apparatus disclosed in Japanese Patent Application Laid-Open No. 2001-324663 (paragraphs 0031 to 0032, FIG. 10, and the like) proposes a structure that protruding portions provided on a fixed barrel and protruding portions provided on a cam barrel are brought into abutment on each other, so a cam follower portion is prevented from being disengaged from a cam groove portion even when an external force is applied thereto.
FIG. 13 is a developed view of an external force absorbing structure, around an optical axis, of the optical apparatus proposed in Japanese Patent Application Laid-Open No. 2001-324663 (paragraphs 0031 to 0032, FIG. 10, and the like). Referring to FIG. 13, a cam groove portion 101a is formed in an inner peripheral surface of a fixed barrel 101. A cam follower 102a, which is provided on an outer peripheral surface of a cam barrel (not shown) provided inside the fixed barrel 101, is engaged with the cam groove portion 101a. With this construction, when the cam barrel rotates in a direction indicated by an arrow of FIG. 13, the cam follower 102a moves along the cam groove portion 101a. Therefore, the cam barrel is driven in the optical axis direction with respect to the fixed barrel 101.
Protruding portions 101b are provided on an object side with respect to the cam groove portion 101a in the inner peripheral surface of the fixed barrel 101. Protruding portions 102b are provided on an outer peripheral surface of the cam barrel. When the cam barrel moves from a retracted lens position toward the object with respect to the fixed barrel 101, the protruding portions 102b move toward the object together with the cam barrel so as not to interfere with the protruding portions 101b of the fixed barrel 101. When the cam barrel is in a zoom region between a wide position and a tele position after having moved toward the object with respect to the fixed barrel 101, the protruding portions 102b is located on the object side with respect to the protruding portions 101b of the fixed barrel 101. When an external force is applied from the object side in this state, the protruding portions 102b comes into abutment on the protruding portions 101b, so the cam follower 102a is prevented from being disengaged from the cam groove portion 101a. 
A lens holding barrel disclosed in Japanese Patent Application Laid-Open No. H10-031149 (paragraphs 0028 to 0029, FIG. 2, and the like) proposes a configuration in which, when an external force is applied to the lens holding barrel, a contact portion provided on the lens holding barrel at an end in the optical axis direction thereof comes into abutment on a cam face of a cam groove portion. A cam follower portion is thereby prevented from being disengaged from the cam groove portion.
However, in the optical apparatus proposed in Japanese Patent Application Laid-Open No. 2001-324663 (paragraphs 0031 to 0032, FIG. 10, and the like), it is impossible to ensure a sufficient circumferential length allowing the protruding portions 102b on the cam barrel to abut on the protruding portions 101b on the fixed barrel 101 (hereinafter referred to as abutment length) in the vicinity of the wide position in the zoom region. This causes a problem in that the vicinity of the wide position is weak in strength against an external force in comparison with the other portions in the zoom region.
When an attempt is made to increase the circumferential length of the protruding portions 102b on the cam barrel to increase the abutment length for abutment on the protruding portions 101b of the fixed barrel 101 in the vicinity of the wide position, the protruding portions 102b interferes with the protruding portions 101b when the cam barrel is driven between the retracted lens position and a position on the object side. Alternatively, it is also conceivable to adopt a method of rotating the cam barrel by a larger angle before its arrival at the wide position after its movement from the retracted lens position toward the object side so as to increase the abutment length of both the protruding portions 102b and 101b in the vicinity of the wide position. In this method, however, the entire circumferential length of the cam groove portion 101a is increased, which leads to the necessity of increasing the diameters of the cam barrel and the fixed barrel. As a result, any effort to reduce the size of the optical apparatus is hampered.